Hydraulics are a good idea. There are moped levers that should push a little extra fluid, but would be designed for flat bars.
I would assume that not all brands and models have identical master/slave cylinders, so you might be able to at least do rudimentary proportioning by choosing a smaller slave cylinder in the rear, and a larger one in the front. And, of course, as
FBinNY mentioned, a small rear disc, and large front disc.
Trickier still, but a tiny air bubble could also reduce the braking force at one caliper.
Then, the kid will have to learn the best way to position his weight on the bike for safe braking.
It is possible that an electric brake could be made in the future. Electric trailer brakes (automotive) have an ingenious feature that the magnet acts on the drum creating drag which in turn activates the brake shoes, and thus creates braking force proportional to speed, and reduces locking.
One could also have dual modal braking on a single wheel if needed (rim + either disc or drum), and also coaster/fixie.